JP2969948B2 - Communication system for matching data packets - Google Patents

Description

Description: FIELD OF THE INVENTION The present invention relates generally to communication systems. More particularly, it relates to communication systems that require timing alignment of data packets.

BACKGROUND ART Communication systems designed to have the property of communicating with many remote subscriber units at short intervals on the same communication channel are called multiple access communication systems. One of the communication systems that can be a multiple access system,
There is a spread spectrum system. In a spread spectrum system, a modulation method is used in which a transmitted signal is spread over a wide frequency band in a communication channel. This frequency band is much wider than the minimum bandwidth required to transmit the information to be sent. For example, audio signals can be sent by amplitude modulation (AM) within only twice the bandwidth of the information itself. Even with other forms of modulation, such as low deviation frequency modulation (FM) or single sideband AM, information can be transmitted within a bandwidth comparable to the bandwidth of the information itself. However, in a spread spectrum system, the modulation of the signal to be transmitted involves capturing a baseband signal (eg, a voice channel) with a bandwidth of only a few kilohertz, and transmitting the signal to be transmitted at frequencies of many megahertz. And distributing over a band. This is performed by modulating the information to be transmitted with the information to be transmitted and the wideband coded signal.

In general, there are three types of spread spectrum communication methods: Direct sequence Modulation of a carrier with a digital code sequence whose bit rate is much higher than the bandwidth of the information signal. Such a system is called a "direct sequence" modulation system.

Carrier frequency deviation in discrete increments within a pattern determined by a hopping code sequence. Such a system is called a "frequency hopper." The transmitter jumps from frequency to frequency within a given set. The order in which the frequencies are used is determined by the code sequence. "Time hopping" and "time-frequency hopping" also have a number of transmissions defined by a code sequence.

Chirp Pulse FM or "chirp" modulation in which the carrier is swept across a wide band during a specific pulse interval.

The information (ie, the message signal) can be included in the spread spectrum signal in several ways. 1
One method is to add information to the spreading code before it is used for spreading modulation. This method can be used in direct sequence and frequency hopping systems. Note that the information sent must be in digital form before being added to the spreading code. This is a module-2 addition to the combination of the spreading code and the information, usually a binary code.
Is included. Alternatively, the information or message signal can be used to modulate the carrier before spreading.

Thus, a spread spectrum system must have two characteristics. That is, (1) the transmitted bandwidth is much larger than the bandwidth or transmission rate of the transmitted information, and (2) the resulting modulated channel bandwidth employing some function other than the transmitted information. To determine.

Spread spectrum systems can be incorporated into multiple access systems in several different ways. One of the multiple access spread spectrum systems is code division multiple access (CDMA).
s) There is a system. CDMA spread spectrum systems
Use direct sequence (DS-CDMA) or frequency hopping (FH-CDMA) spread spectrum techniques. FH-CDMA systems are also based on slow frequency hopping (SFH-CDMA).
And fast frequency hopping (FFH-CDMA). In an SFH-CDMA system, several data symbols representing the sequence of data bits to be transmitted modulate the carrier in one hop. Also FF
In an H-CDMA system, the carrier hops several times for each data symbol.

In the SFH-CDMA system, a plurality of communication channels are created by allocating a wide frequency band portion to each specific channel. For example, communication between two communication units in a specific communication channel is performed by using a frequency synthesizer to generate a carrier in a specific portion of a predetermined wide frequency band in a short time. The frequency synthesizer uses the input spreading code to determine a specific frequency from a set of frequencies within a wide frequency band in which a carrier is generated. The spreading code is a spreading code generator
Is input to the frequency synthesizer. The spreading code generator is clocked or stepped periodically through different transitions. As a result, different or shifted spreading codes are output to the frequency synthesizer. To that end, when the spreading code generator is periodically clocked, the carrier is frequency hopped or reassigned to a different part of the frequency band. In addition to hopping,
The carrier is modulated by data symbols representing the sequence of data bits to be transmitted. A common type of carrier modulation used in SFH-CDMA systems is M-ary frequency shift keying (MFSK).
ying), and the k = log ₂ M data symbols are used to determine which of the M frequencies will be transmitted.

A plurality of communication channels are arranged using a plurality of spreading codes, and frequency bands are allocated to different channels at the same time. As a result, the transmitted signal is in the same wide frequency band of the communication channel, but within a unique portion of the wide frequency band assigned by a unique spreading code. Preferably, these unique spreading codes are orthogonal to each other and the cross-correlation between the spreading codes is substantially zero. The spread of the signal representing the sum of the signals in the communication channel is canceled by a spreading code related to a specific transmitted signal to be recovered from the communication channel (despreadin).
g) thereby recovering the particular transmitted signal from the communication channel. Further, when the spreading codes are orthogonal to each other, the received signal can be correlated to a particular spreading code, enhancing only the desired signal for the particular spreading code and enhancing other signals. You can keep it.

As CDMA technology becomes incorporated into the next generation of cellular systems, the nature of cellular systems adds to the complexity of real systems. For example, soft handoff (soft handoff channel switching)
In a cellular system that incorporates, the synchronization of transmitted frames is important for proper operation. During soft handoff, a mobile station with diversity reception receives voice or control transmissions from two base stations.
Depending on the strength or quality of the transmission by any of the base stations,
The mobile station chooses the transmission of the base station with the best signal quality. Such a structure of a cellular system requires that two base stations transmit the same voice or control data at the same time so that the mobile station can perform diversity on both signals from both base stations.

The process of data packet synchronization must also minimize the packet delay with respect to the air framing boundary to minimize the overall packet delay in the system. The synchronization process is more difficult in nature because the base stations are usually at different distances from a central data distribution point (often a switch). To synchronize transmissions accurately, it is necessary to compensate for differences in the distance of the link or trunk connecting the central data distribution point to the base station. The usual method is to calculate the delay from the central data distribution point to each base station, thereby delaying the data packet to be transmitted. However, this method has some major disadvantages. First, calculating delay is a intensive operation or measurement that consumes valuable processor time during handoff. More importantly, the delay difference between the cell and the central data distribution point can be as large as hundreds of microseconds. Considering the real-time processing environment for processing the packet delay operation (stamping outgoing dummy packets and monitoring the arrival time message of the base station), the uncertainty of the calculation response is They are almost the same size. For this reason, in the case of a discrepancy caused by the uncertainty of the calculation response, it is necessary to add the entire packet delay (20 msec).

This creates a need for a data packet matching method that allows finer packet matching and is less computationally intensive.

SUMMARY OF THE INVENTION A communication system having at least a plurality of transmitters transmits a first packet of data on a first transmitter, transmits a second packet of data on a second transmitter, and transmits the first and second data.
Align packet data to facilitate synchronous transmission.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 generally illustrates a communication system that advantageously employs transmission synchronization according to the present invention.

FIG. 2 generally shows a block diagram of a base station incorporated into the communication system of FIG.

FIG. 3 generally illustrates the air-frame delay between two base stations.

FIG. 4 generally illustrates the aerial frame matching of the delay shown in FIG. 3 in accordance with the present invention.

FIG. 1 generally illustrates a communication system according to the present invention. This communication system is a cellular radiotelephone system having base stations 130-134 coupled to an EMX switch 120. The EMX 120 operates as an interface between the local public switched telephone network (PSTN) and the base stations 130-134 of the cellular radio telephone system. EMX120 is made by Motorola
Motorola Instruction Manual No. 68P81054E issued by Motorola Service Publications, Schaumburg IL
It may be of the type described in 59. Since the subscriber, i.e., mobile station 125, travels throughout the silent telephone system, the mobile station and its corresponding base station--base station 130 in FIG.
−—A communication handoff (channel switching during a call) is required. In the preferred embodiment, the mobile station
125 has a diversity function so that transmissions from two different base stations can be received at once.
After receiving both transmissions, the mobile station 125 determines which of the two transmissions has the best signal quality. The ability to select the transmission with the best signal quality in real time gives the mobile station the ability to perform a soft handoff. This soft handoff process can only be successful when both base stations transmit the required data packets at exactly the same time. In the preferred embodiment, the data packets to be matched include voice data, but the matching technique works equally well with other types of data, such as control data. In addition, the mobile station can communicate with the target base station 130-134.
May use a procedure called mobile-assisted handoff (MAHO) that does not require the use of a scan receiver. Base station 130 supported by mobile station
The quality of the communication decreases between the mobile station and the corresponding base station 130 as the distance from the mobile station increases. When communication drops below an acceptable level, mobile station 125 will send all target base stations 131-134.
Is sent, and the corresponding base station 130 is asked to determine whether any one of the target base stations 131 to 134 is a candidate for handoff. The mobile station measures the signal quality information signal or signal channel transmitted by each of the target base stations 131-134 and generates a signal quality value for each measured signal channel.
The signal channels transmitted by each of the target base stations 131-134 are at different frequencies. At this point, the mobile station can either send the measured signal quality value back to the corresponding base station 130 and proceed with the process, or make its own handoff decision based on the value. The signal quality value measured by the mobile station 125 is a received signal strength indicator (RSSI: rec) of each signal channel of the target base stations 131 to 134.
eived signal strength indications). The RSSI measured for each signal channel represents the signal strength of that particular signal channel at that particular frequency. RSSI is used to facilitate handoff within a cellular radiotelephone system, as well as the diversity feature of mobile station 125.

FIG. 2 generally shows a block diagram of base stations 130-134 used to implement the present invention. By way of example, FIG. 2 shows a base station 130. Interface 205
Connects the base stations 130 to 134 to the EMX120. Interface 205 is coupled to processor 210, which in the preferred embodiment is a Motorola 56001 digital signal processor (DSP). Processor 210 is also coupled to memory block 220, which includes RAM and ROM. Processor 210 is coupled to transmitter / receiver 215, which provides an interface between processor 210 and the channels transmitted by base stations 130-134. EMX120 to base station 13
Packets of compressed audio data entering 0-134 are input to interface 205 and sent to processor 210. The processor 210 converts the packet of the compressed voice data into an inter alia (inter alia) necessary for the aerial frame.
Processes by performing forward error correction (FEC), pinching and periodic redundancy check (CRC).
Thereafter, the processed aerial frame is awaited to be transferred to transmitter / receiver 215 in memory 220 and transmitted aerial to mobile station 125 at the appropriate time.

FIG. 3 generally illustrates the aerial frame delay between two base stations that advantageously employ the present invention. In a preferred embodiment, the EMX 120 includes an inter aria, a vocoder 315, switching hardware 320 coupled to the PSTN, and a processor 312. In future cellular radiotelephone system embodiments, vocoder 315 may be physically separated from EMX120. In addition, vocoder
315 is used to compress voice data from the PSTN into packets of compressed voice data that can be transmitted over links 110, 112 via switching hardware 320. When transmitted by the vocoder 315, a packet arrives at a cell in a compressed voice format at any time with respect to the aerial frame reference 300. After processing the packet of compressed voice data (creating an aerial frame for transmission), the base stations 130 and 131 transmit
Next, it waits until the prepared aerial frame period. Aerial frame reference 300 includes reference stations 131, 131 for frame transmission.
Is a timing reference used by For example, to start an aerial frame transmission, the aerial frame reference
At 300 point A, a strobe occurs, transmission of aerial frame # 1 begins (assume that processing of aerial frame # 1 has been completed before strobe A occurs), and strobe B occurs at point B. Continue until. At this point, transmission of frame # 2 begins and continues until a strobe occurs at point C. This process continues throughout the transmission of the air frame of the audio data.

The resulting delay, which is the source of the problem to be solved, is caused by the difference in length between link 112 and link 110. This in the Figure 3, indicated as △ _L. The packet vocoder 315 of the compressed audio data, because they are transmitted simultaneously to the reference station 130 and 131, packets transmitted to the base station 131 is delayed by a time related △ _L. This time delay is shown as △ _t in the air frame 305 transmitted by the base station 131. A △ _t in the air frame 305 is zero, if the synchronization aerial frame 305 is the air frame 310, transmission takes place in the strobe A aerial frame reference 300, strobe through the frame # 1 of air-frame reference 300 points It will continue until it happens at B. But,
Aerial frame 305 transmitted by the base station 131 is delayed by △ _t, since the processing of the air frame is not completed by the time A to the, to be carried out is matched, the earliest aerial frame transmission can occur Is the aerial frame reference
It becomes # 2 of 300. In the preferred embodiment, each aerial frame (# 0, # 1,...) Of the aerial frame reference 300 and the transmitted aerial frames 305, 310 is 20 times in time.
Milliseconds. Therefore, the base station 131 transmits the aerial frame # 1 of the aerial frame 305 with respect to the aerial frame # 2 of the aerial frame reference 300, and the base station 130 transmits the aerial frame 310.
Is transmitted with respect to the aerial frame # 1 of the aerial frame reference 300, the diversity reception of the mobile station 125 is not performed, and the soft handoff becomes impossible.

△ in order to correct _t, it is preferable to alignment by delaying the air frame 310 transmitted by the base station 130 1 aerial frame only completely. However, in the preferred embodiment, this has several disadvantages. First
The aerial frame 310 is one aerial frame (20 ms)
If the cell is actually used after a handoff, the delay is maintained for that call throughout the call. Second, the vocoder 315 is the base station 1
Since compressed audio frames are output simultaneously for each of 30,131, it is not practical to independently match one of aerial frames 305 or 310 in less than 20 milliseconds.

FIG. 4 illustrates the alignment of aerial frames according to the present invention.
In the preferred embodiment, both aerial frames 305, 310 are aligned by the same amount and the aerial frame reference 30
Allows transmission of a period of zero appropriate aerial frames. Looking at Figure 4, the air frame 400, 405 that are at least △ _t for the time is advanced is shown. However, △
_t is on the order of hundreds of microseconds. Since the end point of each frame # 1 of the aerial frames 400 and 405 to be transmitted is before the strobe occurs at the point A, the strobe occurring at the point A can be used as a trigger strobe for transmission. As a result, each frame # 1 of the aerial frames 400 and 405 to be transmitted is transmitted between the strobe A and the strobe B of the aerial frame reference 300. Aerial frame 400 to be transmitted
By advancing both 405, frame delay △ _t inconsistency occurs in a communication system is solved by adding a delay of a few hundred μ sec in the appropriate cell. Other methods, on the other hand, add a 20 millisecond delay.

In order to correct the delay △ _t is either monitor the size of the frame buffer 323,328 waiting to be transmitted through the air, or timestamp pointer (time
stamped pointer) should be used. If the cell corrects for one structure, the first pointer is stamped when the base station 130 or 131 has finished processing the aerial frame, and the second pointer is the aerial timing strobe in the aerial frame reference 300, e.g. Stamp with A, B, C, etc. In the preferred embodiment, the procedure for advancing or delaying packets of compressed audio data (and processed aerial frames) transmitted from vocoder 315 is in-band or out-of. -Band) by using a vocoder command. The way to advance or delay using vocoder is ETSI / PT1
GSM recommendations 8.60, Version issued by 2
No. 3.2.0 (January 1990). In this method, pulse code modulation (PCM) samples are advanced or delayed before the vocoder by appropriate vocoder commands.

If there is one cell (assuming the time stamp pointer monitoring method is employed), the packet of compressed audio data arrives at any time with respect to the aerial frame reference 300 slot strobes A, B, C, etc. When the processor 210 finishes the aerial frame processing of the compressed audio data, one time pointer is stamped, and when a slot strobe A, B, C, or the like occurs, a second time pointer is stamped. If the difference between the pointers is greater than or equal to the transmit trigger value, eg, half the slot period (10 milliseconds), then the vocoder 315 will receive a packet of compressed audio data from the amount given by approximately 10 milliseconds. Can be ordered to proceed by the reduced amount. If the pointer difference is less than 10 milliseconds, the vocoder 315 can be instructed to delay the packet of compressed audio data by approximately the same amount given by the amount of pointer difference.
For soft handoff (using the time stamp pointer), one of the cells has already been aligned with the aerial frame reference 300. If the difference between the new cell pointers is less than 10 ms, the communication system takes no action. This is because the correct frame is ready to be transmitted over the air before the slot strobes A, B, C, etc. of the aerial frame reference 300. If the difference is greater than 10 milliseconds pointer, the new frame is delayed, the vocoder 315, the instruction is issued approximately equal amounts by (both frames) proceed as in Figure 3 and Figure 4 △ _t.

Continuation of the front page (58) Field surveyed (Int. Cl. ⁶ , DB name) H04B 7/24-7/26 102 H04Q 7/00-7/38 H04J 13/00

Claims (10)

(57) [Claims] 1. A communication system having at least a plurality of transmitters, comprising: a vocoder for encoding a data packet; a first means for transmitting a first data packet;
Second means for transmitting a data packet; and the first means coupled to the vocoder for aligning the first and second data packets with the vocoder and transmitting the first and second data packets. And means for facilitating synchronous transmission by the second means. 2. The data packet according to claim 1, wherein said first and second data packets are one of voice data and control data.
A communication system as described. 3. The communication system according to claim 2, wherein said first and second data packets include the same voice or control data. 4. The communication system according to claim 1, wherein said means for matching to said vocoder further comprises means for temporally matching said first and second data packets to said vocoder. 5. The vocoder has a first data and a second data.
The means for aligning packets in time further comprises means for temporally advancing the phases of the first and second data packets to facilitate transmission within at least one of a plurality of time frames. The communication system according to claim 4, wherein: 6. A communication system having at least a plurality of transmitters for transmitting packets of data in frames having a predetermined time interval, comprising: a vocoder for encoding the data packets; transmitting a first data packet. First means and second means
Second means for transmitting a data packet; coupled to the first transmitting means and the second transmitting means, for determining whether one of the first and second data packets is delayed beyond a transmission trigger value Means for detecting, in response to the detecting means, both the first and second data packets transmitted to the vocoder in time alignment, within the frame. Means for facilitating synchronous transmission. 7. A communication system having at least a plurality of transmitters, each of said plurality of transmitters coupled to a vocoder for encoding a data packet and transmitting in a frame having a predetermined time interval. A communication system comprising: first means for transmitting a first data packet during at least one predetermined time interval of consecutive first and second frames; and at least one of the consecutive first and second frames. Second means for transmitting a second data packet at two predetermined time intervals; coupled to said first and second transmitting means, wherein one of said transmitted first and second data packets is said first data packet; And means for detecting whether it overlaps with the second frame in time; and coupled to the detecting means to advance both the first and second data packets in time. Te, said first frame means to facilitate the transmission synchronization within a predetermined time interval; communication system characterized in that it is constituted by. 8. The communication system according to claim 7, wherein said means for temporally advancing both said transmitted first and second data packets is performed prior to a vocoder. 9. A method for synchronizing transmission of data packets in a communication system having at least a plurality of transmitters, the transmitter transmitting data packets in frames having a predetermined time interval. Encoding the data packet in a vocoder; transmitting the data packet at a first transmitter and transmitting a copy of the data packet at a second transmitter; Detecting whether any one of the packets is delayed beyond a transmission trigger value; and responding to the detecting step, through the vocoder, aligning both of the data packets in time. Facilitating synchronous transmission within said frame by: synchronizing the transmission of data packets. 10. A communication system having at least a plurality of transmitters, wherein the transmitters are coupled to a vocoder and encode for transmitting data packets in frames having predetermined time intervals. A method for synchronizing the transmission of data packets, comprising: transmitting a first data packet from a first transmitter during at least one predetermined time interval of successive first and second frames; Transmitting a second data packet from a second transmitter during at least one predetermined time interval of a first and a second frame; one of the transmitted first and second data packets is the first data packet; And detecting whether or not the first and second data packets overlap with each other in time; and advancing both of the first and second data packets in time with the first frame. How to synchronize the transmission of data packets, characterized in that it is constituted by: the step of facilitating the synchronous transmission during a given time interval.